Isobutene is an important industrial raw material and has recently received attention as a raw material of butyl rubber, polyisobutylene, isobutene oligomer, and methylmethacrylate (MMA). Isobutene is generally present as a mixture of an unsaturated hydrocarbon such as butadiene, N-butene or the like and a saturated hydrocarbon such as N-butane, isobutane, or the like. Thus, to use isobutene as a raw material, isobutene needs to be separated from the C4 mixture.
In general, as a raw material stream for industrial application used to prepare isobutene, C4 raffinate-1 remaining after separation of butadiene by extraction or extractive distillation in a first step of a post-treatment process of a C4 mixture produced by cracking naphtha may be used. C4 raffinate-1 is a hydrocarbon mixture containing olefins, i.e., isobutene, 1-butene and 2-butene (cis and trans) and saturated hydrocarbons, i.e., N-butane and isobutane. Among these materials, 1-butene has small difference in boiling point from isobutene and thus cannot be economically separated from the hydrocarbon mixture by distillation.
As a method of preparing (separating) isobutene from C4 raffinate-1, a t-butyl alcohol (TBA) dehydration method using hydration and dehydration in combination, a methyl t-butyl ether (MTBE) cracking method in which isobutene is obtained by adding methanol to isobutene using an acid catalyst and cracking the methanol-added isobutene, an isobutane dehydrogenation method, or the like may be used.
The isobutane dehydrogenation method generally uses a catalyst such as a Group VIII noble metal, tin, zeolite, or the like. In this regard, when dehydrogenation for preparation of isobutene is performed, isomerization and cracking, which are side reactions, simultaneously occur, and byproducts generated by the side reactions reduce activity of the catalyst, shortening lifetime thereof, and making separation of isobutene difficult. To address these problems, U.S. Pat. Registration No. 4,727,216 discloses a method of minimizing byproduct and easily removing byproduct by using a sulfided type L zeolite catalyst. However, the method is suitable for use in a case in which a large amount of pure isobutane is used as a raw material and thus is not economical and it is deemed that it is impossible to use a great amount of pure isobutane as a raw material in an actual production process.
Isobutene can be easily converted into a derivative thereof by water or alcohol, and thus, the TBA dehydration method and the MTBE cracking method, in which isobutene is selectively obtained from a C4 mixture by addition of water or alcohol, may be generally used as a method of preparing (separating) isobutene from C4 raffinate-1.
The TBA dehydration method is a method in which isobutene of C4 raffinate-1 is subjected to hydration reaction to prepare TBA, the TBA is separated, and then the separated TBA is subjected to dehydration to obtain isobutene. As a hydration reaction method of isobutene, Japanese Patent Application Laid-open Nos. Sho 56-10124, 56-2855 and 55-64534 disclose a method of preparing TBA from isobutene through rechargeable fixed-bed reaction using a strongly acidic ion exchange resin. In the hydration reaction method, a catalyst is fixed and thus separation of isobutene from the catalyst is good, whereas contact efficiency is low and thus reactivity is low. In addition, Japanese Patent Application Laid-open No. Heisei 11-193255 discloses a method of enhancing reactivity of the hydration reaction method and hydration reaction selectivity through use of an appropriate solvent and optimal cycling. However, this method is fundamentally disadvantageous in that hydrocarbon and water as reaction raw materials are not mixed and most of a product needs to be cycled.
In addition, the TBA dehydration to obtain isobutene from TBA generally uses a strong acid such as sulfuric acid or the like as a catalyst, and thus, a manufacturing apparatus that is resistant to corrosion of a strong acid is needed and waste sulfuric acid discharged after dehydration needs to be treated. To address these problems, dehydration may be performed using, as a catalyst, a strongly acidic ion exchange resin containing a sulfonic acid group. In this method, however, when reaction temperature is low, a composition ratio of water after reaction increases and thus reaction rate is significantly decreased. On the other hand, when reaction temperature is high, isobutene as a reaction product is produced in large amounts, which results in production of large amounts of byproducts. With regards to this, U.S. Pat. No. 5,849,971 discloses an enhanced method through use of a reactive distillation column, as compared to existing methods.
The MTBE cracking method is a method whereby MTBE is prepared by etherification of isobutene of C4 raffinate-1 with methanol, the prepared MTBE is separated, and the separated MTBE is subjected to cracking (decomposition) to obtain isobutene. MTBE is a fuel component useful for engines of four-wheel vehicles to increase octane number, and addition reaction of methanol to isobutene in C4 raffinate-1 is substantially faster than addition reaction of water. For this reason, the MTBE cracking method is most widely used for industrial application, and MTBE is widely used as an inexpensive precursor to obtain high-purity isobutene.
U.S. Pat. No. 5,567,860 discloses preparation of MTBE and a method of preparing high-purity isobutene from a C4 mixture through cracking of MTBE. According to the method, MTBE, 2-methoxy butane (MSBE), an unreacted C4 mixture, methanol, water, DME, isobutene oligomer, and the like may be obtained by etherification of a C4 mixture containing isobutene with methanol, and, when the compounds are subjected to fractional distillation using a column, low boiling point materials including C4 hydrocarbon (unreacted C4 mixture), methanol and DME and high boiling point materials including C4 oligomer (isobutene oligomer) may be obtained. MTBE and MSBE are obtained from a side stream taken from the column and cracked through a cracking process using an acid catalyst, thereby preparing isobutene. In the cracking reaction (process), however, main components, i.e., isobutene, N-butene, and methanol are obtained together with non-cracked MTBE and MSBE. Thus, the mixtures need to be separated by distillation, and a C4/methanol azeotrope containing isobutene and N-butene and DME is separated as a low boiling point component. In addition, to obtain high-purity isobutene, the separation process requires washing and distillation one or more times. The method of preparing isobutene from MTBE is complicated in that accompanying materials in the C4 mixture and unreacted reaction products or byproducts obtained through etherification and cracking in plural columns and a washing process have to be removed to obtain high-purity isobutene. For example, DME and C4 oligomer as byproducts have to be treated and methanol and MTBE, which form an azeotrope, have to be recycled to obtain high yield.